The long-term objective of this research is to increase basic understanding of the events of early determination and differentiation in the mouse embryo. Using specifically directed non-H-2 antibodies, the distribution and development variation of non-H-2 antigens in mouse embryos will be studied. The antigens will be quantitated by means of a highly sensitive binding assay, and visualized by means of immunofluorescence and immunoelectron microscopy. Efforts will also center around using non-H-2 antibodies as molecular probes to study cell sorting in chimaeric embryos.